1. Field of the Invention
The present invention relates to a thin-film solar cell array system and a method for producing the same.
2. Description of the Prior Art
Solar cells are components which convert light into electric energy. Usually they comprise a semiconductor material containing n-type and p-type zones, i.e. zones in which current is transported by means of respectively negative and positive charge carriers. These zones are referred to respectively as the emitter and the base. The positive and negative charge carriers generated by incident light are separated and can be conveyed by means of metallic contacts on the respective zones. Only those charge carriers contribute to useable electric power that reach the contacts and do not recombine with a charge carrier of reverse polarity. A further loss mechanism is the reflection of light at the metal contacts, referred to as contact shading the solar cell. The smaller the shading, i.e. the more light which is able to reach the solar cell, the greater the current exploitation by the cell per area and therefore the greater the efficiency. The contacts on the light-facing side, usually the front surface of the cells, are usually designed as comb-shaped structures, so-called grids. However, in order to ensure current conveyance with little resistance, the spacing of the grid fingers must not be selected too large and the number and cross section must not be selected to be too small. A certain amount of shading must be taken into account in conventional solar cells.
With the development of cheaper starting materials, the concept of thin-film solar cells on cost-favorable substrates is gaining more significance. One such known solar cell (see FIG. 1a) comprises only one active solar cell layer 1 comprising a p-type doped base zone 1b and, in the depicted instance of FIG. 1a, n-typed doped selective emitter zones 1a. The active solar cell layer 1 applied to a carrier substrate 2 usually possesses a thickness of approximately 3-50 mm. However, many such substrates 2 are not conductive. Therefore, the electrical contact to base 1b cannot occur from the back surface via the carrier substrate 2. Instead a so-called single-side grid comprising two intermeshing grids, an emitter grid 3a and a base grid 3b, for contacting the emitter 1a and the base 1b respectively must be employed.
Such a solar cell array system can be simultaneously used for connecting multiple solar cells on a carrier substrate as described in DE 197 15 138.
U.S. Pat. No. 4,490,573 describes a solar cell. See in particular the embodiment of FIG. 8, which is provided with a contact layer 51 applied onto a glass substrate 52 and is electrically connected via contacting zones with, for example the n-type doped zone of a solar cell layer 54. Solar-cell layer 54 is produced by means of gradual doping using the doping atoms arsenic and bromide. This is essentially illustrated in FIG. 9 and the respective description. Contact electrodes 16 are placed directly on the p-typed doped layer of the solar cell layer. Therefore, the electrode structures are provided on both sides of the solar cell layer and shade the solar cell layer at least partially from unimpeded irradiation.
The solar cell described in WO89/04062, providing a multilayered solar cell array system, has the same drawback. See in particular the embodiments of FIGS. 1a and b. FIG. 1b essentially shows a first electrode 14 placed directly on a glass substrate on which the silicon-based solar cell layer is applied. A second electrode layer, which is interrupted by dielectric zones 20, is in direct contact with solar cell layer 16. A gridlike electrode structure 32, which is connected to the solar cell layer 16 and to the first electrode layer 14 via the electrically conducting connecting channels 22, is placed on the dielectric layer 20.
FIG. 1b shows a similar known arrangement for a back-contact cell, a concept for highly efficient solar cells. In this case, both contacts 3a and 3b are placed on the back surface of the solar cells to completely eliminate shading on the front surface. If the contacts are realized as narrow grids, light that reaches from the back surface through to the front surface can also contribute to generating electricity (so-called bifacial cell).
Hitherto realization of this single-side grid has only been possible by means of very complicated processes. The selective emitter is created by multiple masking steps: the emitter does not comprise a homogeneous lateral layer but rather comprises a subzone corresponding to the shape of the emitter grid. In this manner, base zones are retained on the surface and can be directly contacted. Placing the respective metal contacts precisely on the corresponding zones poses a critical alignment problem and also requires masks, which must be accurately aligned.
Such a type back-contact cell is described in JP 2-51282 and is known as emitter wrap through (EWT). However, an EWT cell is limited to solar cells made of silicon disks respectively wafers. But the essential cost-saving potential in photo-voltaic cells lies in reducing the use of the expensive silicon to only a few- micrometer-thick layer, so-called thin-layer solar cells. As a carrier substrate is a prerequisite for these thin-layer solar cells, their back surface is not accessible and the known EWT process cannot be applied. At the same time, due to this minimal thickness, thin-layer cells permit using silicon of poorer quality than for conventional or EWT solar cells.
The present invention provides a thin-layer solar cell array system having, placed over a carrier substrate of plane design, a solar cell layer having at least one n-type conducting semiconductor layer zone (emitter) and at least one p-type conducting (base) semiconductor layer zone as well as a first and a second contact electrodes each of different electrical polarity, which are each electrically connected respectively to the emitter and the base in such a way that a solar cell can be produced in a simpler and less expensive manner without impairing the efficiency of the solar cell. Contrary to the state of the art, the electrical contact of the individual semiconductor zones occurs without using masks which require highly accurate alignment and is realizable using simpler methods. Furthermore, the thin-layer solar cell array of the invention permits simple interconnection of multiple solar structures on a carrier substrate as well as allowing provision of a protective diode. In particular, the thin-layer solar cell array system provides the connecting electrodes only on one side of the solar cell layer to, in this manner, have as few as possible shading losses due to the electrode structures. Finally, the invention is a method for producing the novel thin-layer solar cell array system.
The thin-layer solar cell array system of the invention places over a carrier substrate of plane design, a solar cell layer with at least one n-type conducting semiconductor zone, the emitter zone, and at least one p-type conducting semiconductor zone, the base zone, as well as a first and a second contact electrode each of different electrical polarity, which are each respectively electrically connected to the emitter and to the base. The thin layer solar cell is designed in such a manner that the first and the second contact electrodes are applied directly or separated by an electrically insulating layer on the carrier substrate over which an electrically insulating layer is provided with the solar cell layer placed over this insulating layer. The contact electrodes are preferably designed as grid-shaped strip conductors and are made of metal or other highly conductive materials. The two contact electrodes, which are electrically connectable to different polarities, are placed spaced apart on the carrier substrate.
For electrical contacting of the respective semiconductor layer zones (emitter zone, base zone) produced on top of each other with the corresponding contact electrodes, first contact channels and second contact channels are provided which extend through the insulting layer and/or through the active solar cell layer down to the contact electrodes.
Production can occur during deposition of the silicon layer but also by means of transformation respectively by means of diffusion. Preferably multiple first and second contact channels are provided into which electrically conductive material is introduced by means of which the first contact electrodes are electrically connected with the semiconductor zone corresponding to its polarity. Furthermore, the second contact electrode is electrically connected with the semiconductor zone corresponding to the polarity thereof via second contact channels.
The contact channels are designed as one-sided blind holes bordered by the respective contact electrodes and serve to electrically contact the individual semiconductor layer zones without the contacting measures substantially influencing the active solar cell surface. Of the solar cell layer, only the cross sections of the individual contact channels are lost, which however is a relatively small part of the surface.
With the contacting and the layered arrangement of the thin-layer solar cell of the invention, the solar cell can be divided into lateral sections as desired in that the layers can be separated down to the insulating layer covering the contact electrodes. Such subdivision permits combining several solar cells applied to a single carrier substrate. This is not possible, for example, with the previously mentioned EWT solar cells. Furthermore, protective diodes, which are placed on the same carrier substrate as the solar cells, are provided to bridge the solar cells in the event of malfunction and in this way increase secure operation and the lifetime of the solar cells.
Further embodiments of arranging the thin-layer solar cell array system and the method for producing the same are explained in more detail with reference to the drawings.